1. Field of the Invention
The present invention relates to a photoelectric converting film stack type solid-state image pickup device fabricated by stacking a photoelectric converting film on a semiconductor substrate having formed on the surface thereof a signal read circuit.
2. Description of the Related Art
As for the photoelectric converting film stack type solid-state image pickup device, a prototype device is described, for example, in JP-A-58-103165. In this solid-state image pickup device, three photosensitive layers are stacked on a semiconductor substrate, and electrical signals of red (R), green (G) and blue (B) detected by respective photosensitive layers are read out by MOS circuits formed on the semiconductor substrate surface.
Solid-state image pickup devices having such a constitution were proposed in the past and thereafter, CCD-type image sensors or CMOS-type image sensors where a large number of light-receiving parts (photodiodes) are collectively stacked on the semiconductor substrate surface part and color filters of red (R), green (G) and blue (B) are stacked on respective light-receiving parts have made remarkable progress. At present, an image sensor where hundreds of light-receiving parts (pixels) are clustered on one chip is mounted on a digital still camera.
However, CCD-type image sensors or CMOS-type image sensors are reaching the limit of their technical progress and come to encounter a problem of bad production yield because the opening size of one light-receiving part is about 2 μm and approximated to the wavelength order of incident light.
Furthermore, the upper limit of the quantity of photoelectric charges accumulated in one minute light-receiving part is as small as about 3,000 electrons and with this number of electrons, it is difficult to clearly express 256 gradations. Therefore, CCD-type or CMOS-type image sensors can be hardly expected to be more enhanced in view of pictorial quality or sensitivity.
As a solid-state image pickup device capable of solving these problems, the solid-state image pickup device proposed in JP-A-58-103165 is taken notice of, and image sensors described in Japanese Patent No. 3,405,099 and JP-A-2002-83946 are newly proposed.
In the image sensor described in Japanese Patent No. 3,405,099, a medium having dispersed therein ultrafine silicon particles is used for the photoelectric converting layer and by stacking three photoelectric converting layers differing in the particle diameter of the ultrafine particle on a semiconductor substrate, electrical signals according to respective intensities of red, green and blue lights received are generated by respective photoelectric converting layers.
In the image sensor described in JP-A-2002-83946, similarly, three nanosilicon layers differing in the particle diameter are stacked on a semiconductor substrate, and electrical signals of red, green and blue colors detected by respective nanosilicon layers are each read out into an accumulation diode formed on the surface part of the semiconductor substrate.
However, since the ultrafine particle used is silicon in both of Japanese Patent No. 3,405,099 and JP-A-2002-83946, an electron-hole pair generated upon receiving light cannot be satisfactorily prevented from recombining on the ultrafine particle surface and this causes a problem that the performance as a solid-state image pickup device is not satisfied.
On the other hand, studies of ultrafine particles are introduced in B. O. Dabbousi et al., “(CdSe)ZnS Core-Shell Quantum Dots: Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites”, J. Phys. Chem. B 1997, 101, 9463-9475, though this is not related to a solid-state image pickup device. “(CdSe)ZnS Core-Shell Quantum Dots Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites” describes an ultrafine particle obtained by coating the periphery of a CdSe quantum dot with ZnS. This CdSe quantum dot with ZnS shell is advantageous in that the electron-hole pair can be prevented from surface recombination as compared with an ultrafine silicon particle.
In order to practically use the photoelectric converting film stack type solid-state image pickup device, what material is used to form the photoelectric converting film is present as a problem. This problem can be overcome by using a CdSe quantum dot with ZnS shell introduced in B. O. Dabbousi et al., “(CdSe)ZnS Core-Shell Quantum Dots: Synthesis and Characterization of a Size Series of Highly Luminescent Nanocrystallites”, J. Phys. Chem. B 1997, 101, 9463-9475 in place of the ultrafine silicon particle described in Japanese Patent No. 3,405,099 and JP-A-2002-83946.
However, it is necessary to solve another problem that even when a CdSe quantum dot with an ZnS shell is merely dispersed in a medium and formed into a film and the film (photoelectric converting film) is interposed between upper and lower transparent electrode films and applied with a voltage, the photoelectric charges (signal charges) cannot be efficiently taken out.